There already exist known image encoding schemes (JPEG, JPEG-XR) or hybrid video encoding schemes (MPEG, H.264) as well as video data compression techniques. Among these, numerous video encoding techniques use a blockwise representation of the video sequence, for example techniques implementing the video compression standards derived from the MPEG organization (MPEG-1, MPEG-2, MPEG-4 part 2, etc) or l'ITU-T (H.261, . . . , H.264/AVC).
Thus, according to the H.264 technique and as illustrated in FIG. 1a, each image 1 can be sub-divided into slices, themselves sub-divided into macroblocks 10 which are then sub-divided into blocks 11. A block is constituted by a set of pixels.
The encoding of a block is classically achieved by means of a prediction of the block and an encoding of a prediction residue to be added to the prediction. The prediction is made by means of already rebuilt information (already encoded/decoded preceding blocks in the current image, preliminarily encoded images in the context of a video encoding, etc).
In a given encoding scheme, several different encoding modes can be implemented for the encoding of the blocks. An encoding mode generally comprises two phases, a first phase of prediction of the samples to be encoded followed by a second phase of encoding of prediction residues. Typically, the blocks can be encoded by different encoding modes such as intra, inter, skip encoding modes.
For these different encoding modes, the first phase of prediction of the samples typically corresponds to:                a temporal prediction, i.e. with reference to a reference block belonging to one or more images; and/or        a spatial prediction as a function of blocks neighboring the block to be encoded of the current image.        
In this latter case, the prediction cannot be done except on the basis of the blocks which have been previously encoded.
The encoding mode known as the “intra” mode uses only information contained in the image itself. In other words, the prediction of a block of an image encoded in intra mode has recourse to previously encoded neighboring blocks of the same image. For example, a current block is encoded by means of a value of texture of the already encoded/decoded neighboring blocks.
The encoding mode known as the “inter” encoding mode uses a prediction by means of motion compensation from previously encoded images. More specifically, this type of encoding consists in considering one (or more) reference images. A shift or motion between the reference image and the current image is set up for a block to be encoded of the current image. The block used for predicting the block to be encoded is the block of pixels of the reference image shifted by the value of the motion vector.
The encoding mode known as the “skip” mode is a particular mode of “inter” encoding and achieves a temporal prediction for which no information is transmitted to the decoder. In other words, it is possible to “skip” a block if pieces of basic encoding information have already been determined for this block. In this encoding mode, the prediction is done using a motion compensation of the current block by means of the motion vectors of the neighboring blocks if they exist in the reference image, and no prediction residue is encoded or decoded.
For a given encoding mode, prediction parameters are then established, and then encoded. For example, according to the H.264 technique, it is possible to encode prediction parameters for each block, such as the encoding mode (intra, inter, skip), the type of partitioning, information on prediction (orientation, reference image etc), motion information (motion vector), texture information (direction of extrapolation of the texture values), encoded coefficients etc.
According to the H.264 technique, images I are encoded by spatial prediction (intra prediction), images P and B are encoded by temporal prediction relatively to the other images I, P or B encoded/decoded by motion compensation.
During the encoding of these prediction parameters (for example the motion vector of the block), in order to reduce their encoding cost, their value is predicted from the values of the same prediction parameters for the already encoded neighboring blocks, and having the same encoding mode (for example the motion vectors of the neighboring blocks).
For example, the motion vector used on a block encoded in “inter” mode is encoded by means of a predictive encoding such as the following:                at a first stage, a prediction vector for the motion vector of the block considered is set up. Typically, such a vector, known as a median vector, is defined from the median values of the components of the motion vectors of already encoded neighboring blocks;        at a second stage, the prediction error, i.e. the difference between the motion vector of the current block and the previously established prediction vector is encoded.        
An extension of this technique of motion vector prediction is proposed by J. Jung and G. Laroche in the document <<Competition-Based Scheme for Motion Vector Selection and Coding>>, ITU-T VCEG, AC06, July 2006.
This technique consists in placing several predictors or prediction candidate vectors (beyond the median predictor used by AVC) in competition and indicating which vector, of the set of prediction candidate vectors, is the vector effectively used.
However, with this technique of encoding by competition, the definition of this set of predictors is difficult. Indeed, the fact of increasing the number of these candidates (through a greater set) gives a better prediction but has the detrimental effect of a higher cost of indicating the predictor to be used.